The disclosed invention generally relates to metal gate metal-oxide-silicon (MOS) capacitors, and is particularly directed to a process for making metal gate MOS capacitors with standard silicon gate processes. The metal gate capacitors made in accordance with the invention have capacitance values which are reliably controlled in manufacture and which have reduced variation with voltage.
In integrated circuits made with known silicon gate MOS processes, capacitors are formed during the process of forming other elements, such as transistors, by application of selected ones of the process steps to the areas that are to include capacitors. Particularly, with known silicon gate processes, a capacitor may be implemented as a "polysilicon gate capacitor" or as a "polysilicon to polysilicon gate capacitor."
An important consideration with a polysilicon gate capacitor is the need for an extra highly doped N+ or P+ region, which also requires additional process steps. Also, the capacitance value of a polysilicon gate capacitor is that its capacitance value is limited by the particular process utilized to provide the gate oxide that forms the dielectric of the capacitor, unless additional processing steps are utilized. Furthermore, the capacitance of a polysilicon gate capacitor varies considerably with the voltage applied across its terminals.
An important consideration with a polysilicon to polysilicon gate capacitor is the need for an extra layer of polysilicon. Further, the capacitance value of a polysilicon to polysilicon gate capacitor is difficult to control due to variation of polysilicon oxide thickness. Also, a polysilicon to polysilicon gate capacitor has the poor oxide dielectric strength due to polysilicon asperities.
In integrated circuits made with known metal gate processes, capacitors are also formed during the process of forming other elements by application of selected ones of the process steps to the areas that are to include capacitors. However, with metal gate processes the source and drain regions are typically implanted prior to metallization of the gates which presents alignment problems.